(1) Field of the Invention
The present invention relates to an image forming apparatus, such as a copier and a laser printer, that forms a color image by superimposing images formed in different colors, and particularly relates to an improvement in a technique for adjusting a deviation of an image forming position for each color.
(2) Description of Related Art
In a conventional color image forming apparatus, data of an image to be reproduced is separated into four reproduction colors cyan, magenta, yellow, and black. Toner images corresponding to these reproduction colors are respectively formed and sequentially transferred onto a recording material such as a recording sheet. Here, the toner images are superimposed on the recording material to form a color image. When the toner images are deviated from the respective correct image forming positions, color deviations occur, thereby causing deterioration in the image quality. Hereinafter, the reproduction colors are respectively referred to as C, M, Y, and K, and components related to these colors are assigned numerals with a corresponding C, M, Y, or K.
A so-called "tandem-type" image forming apparatus, as one example of a conventional color image forming apparatus, has image forming units set in line corresponding to reproduction colors in a transporting direction of the recording material transported by a transfer belt. A timing at which a toner image is formed is different between the image forming units. Thus, the color deviations are likely to occur in the tandem-type image forming apparatus. It has been one of the greatest challenges to find a way to reduce the color deviations.
In general, registration adjustment is performed to adjust image forming positions in the tandem-type image forming apparatus, thereby preventing the color deviations from occurring. For the registration adjustment, toner patterns in a predetermined shape are formed on the transfer belt at predetermined positions, with the transfer belt being driven to move in the transporting direction of the recording material. The toner patterns are detected by an optical sensor set further downstream than the image forming units in the transporting direction of the recording material. The deviations of the toner patterns from the predetermined positions are obtained from the detection results given by the optical sensor. Then, the image forming positions are respectively adjusted in accordance with the detection results. Hereinafter, the toner patterns used for the registration adjustment are referred to as the "registration patterns". In general, a charge coupled device (referred to as the "CCD" hereinafter) or a photo diode (referred to as the "PD" hereinafter) is used for a photoreceptor of the optical sensor, and a light emitting diode (referred to as the "LED" hereinafter) is used for a light emitter of the optical sensor.
To properly maintain the quality of reproduced images by means of the registration adjustment, forming positions of the registration patterns should be detected with extreme precision from a few micrometers to less than twenty micrometers.
As a method for detecting the forming positions of the registration patterns formed on the transfer belt, a toner density on a surface of the transfer belt which is being rotated is sampled by the optical sensor that has the PD as the photoreceptor. When a line in a registration pattern passes by the optical sensor, a sampling waveform is obtained as shown in FIG. 1A. Here, a barycenter of this sampling waveform may be determined as the forming position of the line. In this method, clearances between sampling points are interpolated as shown in FIG. 1A to obtain the barycenter. Therefore, the number of sampling points can be reduced, and also, the forming position of the line in the registration pattern can be determined with high precision regardless of the sampling timing.
However, when values outputted by the optical sensor as the sampling points on detecting a registration pattern are small as shown in FIG. 1B, the sampling waveform cannot be clearly recognized. Also, adverse effects caused by noises and the like increase. To address this problem, methods for raising the output values have been suggested. For example, a background level of the transfer belt, on which the registration patterns are formed, can be made uniform. Or, the intensity of the laser beams can be maximized when the registration patterns are formed, so that part on the transfer belt where the registration patterns are formed is a vivid contrast to part where they are not formed.
Meanwhile, when the optical sensor having the LED as the light emitter is used, the reflectivity and transmittance are different among the toner colors C, M, Y, and K. For this reason, a detection level varies among the registration patterns formed in the corresponding toner colors. If the background level of the transfer belt is made uniform, or if the contrast on the transfer belt is enhanced, the detection level is raised for every color. Therefore, the variations in detection level cannot be eliminated.
As one example, suppose that the detection levels are greatly different among the four colors. When the output values of a color whose detection level is the lowest are increased to appropriate values by means of the uniformalization of the background level or the contrast enhancement, the output values of a color whose detection level is the highest are also increased. The increased values of the color with the highest detection level may reach the detection limit and may be outputted as constant values around the peak value (barycenter) as shown in FIG. 1C. In this case, the waveform cannot be correctly obtained. This leads to the deterioration of the detection precision. Similarly, when the output values with the highest detection level are reduced to appropriate values, the output values with the lowest detection level become too small. This also leads to the deterioration of the detection precision.
Meanwhile, a method to adjust the output values of the four colors has also been suggested. More specifically, a source of light is switched by the light emitter of the optical sensor for each of red (R), green (G), and blue (B). Then, the registration patterns for C, M, and Y are exposed using the corresponding complementary color lights. Using this method, however, the source of light is increased in size and exposure positions are set different among the colors. To detect the same point by each source of lights, the optical sensor needs to be increased in size and the high precision is required. This leads to a high cost. In addition, the values outputted by the optical sensor on detecting the registration pattern formed from black toner containing carbon cannot be raised using this method.